The problem being addressed by the invention is how to interrupt the data path in a terrestrial communications network by inserting a satellite communications link, without disturbing the timing and control signaling between the sending portion and the receiving portion of the terrestrial network. Typical high speed data communications networks require precise timing in order to enable the multiplexing and concentration of many transmitting sources at the transmitting end and the demultiplexing and time expansion of the messages received over the network at the receiving end. A further problem arises in connection with multiplexed and time concentrated data communications when it is desired to maintain duplex communications links, wherein each respective end of the communications link can simultaneously carry on transmission and receiving operations. In order to maximize the efficiency of the communications network, the transmitted messages and received messages for a particular member of the communications network should occur synchronously at the multiplexing and demultiplexing sites, in order to enable a single switching operation to convey both the transmitted and the received messages in the duplex mode. A corollary problem involved is that the satellite communications network, although synchronous within itself, is not necessarily synchronous with the terrestrial network. Some means must be provided to compensate for moments of data traffic surplus or moments of data traffic deficiency for messages arriving at the interface between the terrestrial network and the satellite link, so that similar compensatory steps can be carried out at the opposite end of the satellite link, thereby enabling the timing environment of the transmitting portion of the terrestrial network to be accurately reconstructed at the receiving portion of the terrestrial network.
An example of a time domain multiple access (TDMA) satellite communications network within which the invention disclosed herein can find application, is described in U.S. Pat. No. 4,320,504 to Alvarez, et al entitled "Mechanism for Synchronization of Data Ports in TDMA Communication," which is assigned to the IBM Corporation. A typical communications protocol which can be used to connect the satellite network to a terrestrial data communications network is the standard X.24 interface as defined by the Consultative Committee for International Telephone and Telegraph (CCITT) recommendation X.24, as amended in Geneva, 1976. It is desirable to be able to characterize such an interface from the point of entry into a satellite communications network to the point of exit, so as to render the characteristics of the satellite network transparent to the users of the terrestrial network connected to it.